<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>输入圆的半径计算面积和周长</title>
</head>
<body>
  <div>圆的半径：<input id="r" type="text"></div>
  <div>圆的周长：<input id="cir" type="text"></div>
  <div>圆的面积：<input id="area" type="text"></div>
  <script>
    var r = prompt('请输入圆的半径');
    if(!isNaN(r)){
      var cir = 2 * Math.PI * r;
      var area = Math.PI * r * r;
      document.getElementById('r').value = r;
      document.getElementById('cir').value = cir.toFixed(2);
      document.getElementById('area').value = area.toFixed(2);
    }else{
      alert('半径错误')
    }
  </script>
</body>
</html>